German Open Application DE-OS No. 28 49 838 discloses a screening surface for a screening machine which is composed of a plurality of individual synthetic resin sieve elements or units assembled together in side-by-side relationship. The peripheral edge of each sieve unit is of much greater thickness than the screening surface or layer inwardly of this edge. The sieve surface has downwardly extending or projecting reinforcing ribs. The sieve surface, where it is thickened, forms substantially a rigid element and tends to vibrate like the edge of the element under the screening action generated in the screening machine. It is also known to reinforce the edges of a screen with reinforcing bars, e.g. of iron.
The dry screening of fine-grained materials, for example, crushed sand, with such screening elements or units can be effected in rainy seasons and periods of high atmospheric humidity only with relatively great difficulty. The mineral meal which is formed in the crushing process picks up moisture rapidly and tends to agglomerate and plug the openings of the screening surface. To prevent caking-up on the screening surface, a number of approaches have been proposed.
It has been suggested, for example, to provide the screening machine with crossbars which alternately compress and stretch the screening surface which, for that purpose, may be sufficiently flexible to enable it to sag between the crossbars.
Alternatively, where rod screens are used, i.e. so called harp screens, in which the individual wires develop a natural vibration in addition to the vibration imposed by the machine and because of this vibration keep the screen lining free from clogging, the wear of such screens is comparatively high.
Still another approach has been to provide plastic screens in which the individual screen openings are not made as square, round or rectangular holes, but rather are given a U-shape with a lip intended to form a natural vibratile element performing the function of the vibrating wire rod previously described and thus tending to free the screen surface of adherent material. The result of this vibration of a portion defining the screening opening, however, is a constant change in the dimensions of the opening and thus in the sieve size. The result is inaccuracy in the separation range or limit.
It has also been suggested to provide the screen layer as a plastic, thin perforated material which is fixed on a support only along its outer edges and otherwise rests loosely on the substructure of the screening machine. The vibrational movement of the machine here tends to cause movement of the looser areas of the screening material which repeatedly knocks against the understructure. The result is a fluttering movement of the material which is intended to bring about a self-cleaning effect. In many cases, however, the energy applied by the machine is inadequate to cause the material to undergo a fluttering movement. The fluttering movement also results in the application of bending stress to the material and material fatigue can develop therefrom so that premature breakage of the material may occur.
Finally, mention may be made of the use of rapping devices of various types, these devices being located beneath the screen lining and tend to counter the obstruction of the screening surface caused by the accumulation of moist fine material which is screened.
German Open Application DE-OS No. 22 40 051 also describes a screening surface which is maintained under tension. The maintaining of such a screening surface is difficult, complex and expensive. For the fabrication of such screening units, it has been proposed to tension a rectangular screen layer elastically by drawing opposite edges thereof apart mechanically and then to affix this screen layer to a rigid frame by cementing, nailing, screws or like means. This arrangement only results in the elastic elongation of the center zone perpendicular to the lateral edges of the screening surface which is formed.
The further one goes from the central region, the less is the tension. With tension elements such as round-section rods on all four sides, the sieve fabric can be stretched, but it is not possible with this approach to ensure a uniform distribution of tension over the entire surface.
The mechanical tension can be applied by levers or like mechanical mechanisms and pneumatic or hydraulic means, using clamping elements which engage limited areas along the outer edges of the screen layer and tug these edges outwardly. Not only is this a complicated and time-consuming process, but the apparatus required is expensive and it is not always possible to ensure a uniform tension. The clamping elements require over-dimensioning of the screening surface and frequently the screening layer must extend beyond the periphery of the frame and thus an excess must be cut off in a time-consuming process.